This invention relates to electrodes for electrochemical processes and to electrochemical cells and has particular reference to hypochlorite cells operating at low temperatures and to zinc winning cells. It is well known to make an electrode for use in an electrochemical cell from titanium with an anodically active coating. Titanium is chosen for its corrosion resistance which is related to the formation of an adherent oxide film on the titanium surface. The oxide film prevents a corrosion attack on the substrate titanium metal itself when the electrode is in use. Conventionally the titanium substrate is coated with a layer of a platinum group metal which forms an anodically active coating. The term "platinum group metal" as used herein is intended to cover metals chosen from the group platinum, iridium, palladium, rhodium, ruthenium and alloys thereof.
Although the presence of the oxide film on titanium will substantially increase the resistance to corrosion of the material, there are circumstances in which the titanium can corrode when operated as an anode with an anodically active coating on its surface. In these circumstances the anode tends to fail by reason of detachment of the anodically active surface and its falling off the anode rather than electrochemical wear of the anodically active material itself. Two particular applications for anodically active coated titanium in which this detachment is a problem are:
1. the operation of a hypochlorite cell at low temperatures (below 10.degree. C.); and PA0 2. the use of an anode in zinc winning from a zinc sulphate solution. PA0 i. forming a layer of an oxide of a metal chosen from the group titanium, tantalum, zirconium, hafnium and niobium on the titanium surface, PA0 ii. heat treating the layer in a vacuum or in a non-oxidising atmosphere, said atmosphere being substantially hydrogen-free, at a temperature and for a time sufficient for the titanium partially to reduce the oxide, PA0 iii. applying to the oxide layer a layer of an anodically active material. PA0 i. applying a paint containing an organic or inorganic compound of the platinum group metal(s) to the surface and heating in air or an oxygen-containing atmosphere at a temperature in the range 350.degree. C. to 650.degree. C. to convert the compound to the metal(s) or oxide(s), PA0 ii. electroplating the platinum group metal onto the oxide layer or onto a previously applied painted and fired platinum group metal layer.
As will be explained in more detail below there are particular problems associated with the operation of hypochlorite cells at temperatures below 10.degree. C. and also there are problems in providing an economically viable anode for use in metal winning operations where such anode is based on a coated titanium substrate.
The present invention is concerned with an electrode which has improved operating characteristics under the circumstances where the anodically active material is liable to become detached. It should be pointed out that in many cases it is not understood why the anodically active material becomes detached nor why the invention as set out below leads to an improvement in properties of the electrode.